1. Field of the Invention
The present invention relates to a method and an apparatus for vulcanising pneumatic tyres.
2. Description of the Related Art
Generally, in the manufacture of pneumatic tyres for vehicle wheels it is provided that a moulding and vulcanisation treatment be carried out subsequently to a step of building the green tyre through assembly of the respective components, each of which has its own elastomer composition, and some of which are equipped with suitable reinforcing structures; said treatment aims at determining the structural stabilisation of the tyre through cross-linking of said elastomer compositions and at the same time, as generally required, enables formation of a desired tread pattern thereon as well as of possible distinctive graphic marks at the tyre sidewalls.
To this aim, the green tyre is introduced into a suitably heated vulcanisation mould having a moulding cavity of a shape conforming to the final shape to be given to the tyre itself.
After closure of the mould has occurred, the green tyre is pressed against the holding walls of the moulding cavity while simultaneously the necessary heat to carry out vulcanisation of the tyre itself is supplied. For the purpose, a bladder of toroidal conformation for example, is caused to expand within the tyre through admission of steam under pressure into said bladder, so as to bring the latter into contact with the inner surface of the tyre and compress said tyre against the holding walls of the moulding cavity.
The steam under pressure admitted to the expanded bladder within the tyre is also used to supply part of the necessary heat for vulcanisation. Another heat portion is supplied through the mould from the outside of the tyre, suitably heated by means of pipelines for circulation of steam or other heating fluid that are arranged in the vulcanisation apparatus.
Usually, the steam-supply temperature and pressure and the residence time of the tyre in the vulcanisation mould are managed following a pre-established program, obtained based on experimental data, so as to bring the tyre components having different elastomer compositions to a desired cross-linking degree.
When vulcanisation has been completed, supply of heat is stopped and the mould is opened to enable removal of the tyre and prepare the mould to a new moulding/vulcanisation cycle.
By so doing however, phenomena of over-vulcanisation and/or insufficient vulcanisation of the tyre or parts thereof can easily occur. These phenomena can, for example, take place following variations in the temperature of the steam used to heat the moulds. In addition, variations in the mould temperature can also occur, due to variations in the ambient temperature for example, or in the temperature of the heating fluids and the amount of heat dissipated by the mould in the time unit and/or in the opening periods of same between the end of a vulcanisation step and the beginning of vulcanisation on a subsequent tyre.
In an attempt to eliminate the above described problems, U.S. Pat. No. 3,718,721 proposes a method of controlling the vulcanisation state of at least one portion of a tyre during supply of heat to the same, according to which a probe for temperature detection is introduced into a predetermined tyre region.
During supply of heat to the tyre, detection of the temperature of the elastomer material in relation to time is carried out close to the probe, to calculate the true vulcanisation state reached by the tyre portion where the probe is inserted. On achievement of a predetermined cross-linking degree, supply of heat is stopped.
According to the teachings of U.S. Pat. No. 3,718,721, the probe for measurement of the temperature to be detected is disposed in a tyre region where heat transmission takes place with great difficulty, or in any case where the lowest cross-linking degree is expected to be reached at the end of the process. Should the presence of several probes at different tyre regions be provided, interruption of heat supply will be carried out based on the data sent by the probe detecting the lowest cross-linking degree. Bringing said tyre region to the right cross-linking degree will guarantee a sufficient vulcanisation also of the remaining tyre portions.